Soybean is probably the best protein source among possible plant feed stuffs available having reasonable amino acid profiles that could be used as an alternative to animal protein. Soy protein products are used in various kinds of foods such as infant formulas, soups, meat analogues, tofu, frozen foods, etc. Soy protein products are also used to improve texture of meat products, increase the protein content of the food, to enhance moisture retention and also as an emulsifier. The major factors that drive the soy protein market are functionality, health benefits, environment friendliness, cost and versatility. Among all plant proteins, soy protein is the most similar to the animal protein in terms of amino acids and soy protein is much cheaper than animal protein. Worldwide soy protein market in terms of revenue had a worth of $5 billion in 2011 and is expected to reach $9 billion by 2017. Asian market is expected to be the fastest growing market in upcoming future due to its advancement and increasing demand from China and India. North America especially United States dominates the global soy protein market right now, accounting for more than 35% of the global soy protein market demand in 2012.
Successful inclusion of soy proteins into various animal feed or human food products such as aquaculture feed for fish and shrimp, poultry feed, and meat, typically requires the proteins to exhibit similar characteristics to those of the proteins being replaced or supplemented in the feed or food. Different functional properties of soy proteins are required in varying degrees to be included in different kinds of food. And the functionality is affected by the methods used to make the soy protein products.
Many plant seeds, like soy beans, wheat, pulse, peas, or chickpeas, contain a significant amount of usable protein, together with some fats and both soluble and insoluble carbohydrates. Soybean meal, for example, contains approximately 30-35% carbohydrates, mostly non-starch polysaccharides (NSPs) and oligosaccharides. The soluble carbohydrates and the fats can easily be removed by conventional methods, leaving the protein material and the insoluble oligosaccharides and polysaccharides. These oligo- and, particularly, poly-saccharides have been found to lower digestibility of soy products when included in animal feed.
Processed defatted soybean products are typically divided into three categories: soy flour, Soy Protein Concentrate (SPC) and Soy Protein Isolate (SPI). As used herein, soy flour is a powdery defatted soybean meal containing about 50% protein. As used herein, SPC is any soybean meal products processed to have protein contents higher than that of soy flour but lower than that of SPI. The typical SPC has protein content in the range of 65% to 75%. As used herein, SPI refers to any soybean meal products processed to have a protein content of at least 90%. As used herein, the term “soybean hulls” refers to the soybean seed coats, which are removed and collected while soybeans are processed for soybean oil and soybean meal products. As used herein, “soybean flakes” refers to the soybean meal products processed into a physical shape of flakes. As used herein, “soybean powder” refers to the soybean meal products processed into a physical shape of powders.
Current commercial processes for producing SPC and SPI use non-enzymatic methods. SPC is currently produced by washing soy flour with water, often containing a pH buffer and/or an organic solvent. The water soluble carbohydrates (and other soluble and colloidal materials) are removed from the mixture to leave the protein-rich solids that remain. SPC thus produced usually has a protein content of about 65-70% but retains the insoluble soy carbohydrates including the non-starch polysaccharides (hemi/cellulose and pectin), and lignin. The protein yield (i.e., the portion of initial proteins retained in the product) for SPC production can be high (90%-98% depending on the method), but it is less ideal for many uses because of the presence of the hard to digest polymers.
SPI, on the other hand, is currently prepared by first dissolving proteins in aqueous solutions, together with water soluble carbohydrates and others. This causes disintegration of soybean meal particles and allows removal of insoluble constituents by centrifugation. Proteins in the supernatant are then made insoluble, for example by adjusting the pH with acid to a particular pH where the proteins precipitate out and collected by centrifugation. The SPI product thus prepared has higher protein content (about 90%) but is costly to produce and gives a dry weight yield of only about only 30% (protein yield about 60%).
Moreover, many SPC or SPI products are also currently made using methods that include alcohol leaching or treating with acid. Soy proteins obtained using alcohol leaching have been found to have a much lower nitrogen solubility index (NSI), which results in a lower functionality of the product. This is because the protein has been denatured to a greater extent by the alcohol and the heat used in the alcohol leaching process. Further, during the conventional acid wash or alcohol leaching process, some of the alcohol and/or acid remains in the soy protein precipitates and must be removed by additional processing. This additional processing makes the process less economical or may be sufficiently harsh to reduce the value and protein content of the product.
The high content of non-starch polysaccharides (NSPs) in soybean meal and SPC produced by the current processes as described above is a major challenge to the use of soy products, particularly as animal feeds. It is believed that soybean NSPs in feed may have the effect of increasing the viscosity of the intestinal content, which might also be a reason for poor digestibility of nutrients in these animal feeds. Soybean meal contains almost 15% oligosaccharides and 20% NSPs. Soy protein concentrate (SPC) produced by the current commercial method contains 3-5% oligosaccharides and 14-17% NSPs, which have been reported to be indigestible to fish. Reduced digestibility of fat and proteins in Atlantic salmon, for example, have been reported due to the presence of dietary NSPs in soybean meal. (See Storebakken, T., S. Refstie, and B. Ruyter, Soy products as fat and protein sources in fish feeds for intensive aquaculture. Soy in Animal Nutrition, 2000: p. 127-170, the disclosure of which is incorporated herein by reference in its entirety). These components were reported to be responsible for low growth performance and induced enteritis in several salmonids species fed with soybean meal-containing diet.
Another problem with the conventional production processes is the presence of non-functional indigestible fibers in the final product. Natural soy fiber is derived from the parenchyma cell walls of the soybean. The presence of these fibers can bind with the otherwise digestible proteins to prevent the soy products from being used in various animal feed.
In addition, the prior art processes remove the soluble material from the soybean meal either by washing it with large quantities of water or by treating with acid or leaching with alcohol and as a result, it is often not practical to try to utilize the soluble material for other purposes. The prior art processes do not do anything to break down the soluble oligosaccharides, such as raffinose, stachyose and verbascose, into easily fermentable sugars. These oligosaccharides are not readily metabolizable to many organisms and are a hindrance in many industrial applications. In addition, because neither the soluble and insoluble oligosaccharides nor the polysaccharides are broken down into fermentable sugars and a very large quantity of water may be used to wash the soybean meal, the concentration of fermentable sugars in the wash water is not high enough to justify further processing. Moreover, the large quantity of wash water used likewise makes it difficult to easily recover the soluble proteins. And, as discussed above, where the soy proteins obtained using alcohol leaching have been found to have a much lower nitrogen solubility index (NSI) and where the conventional acid wash or alcohol leaching processes are used, some of the alcohol and/or acid remains in the soy protein precipitates and must be removed by additional processing.
Therefore, there is a need in the art for more economic processes for producing soy products having a high protein content and low level of indigestible components, while at the same time generating and capturing the monosaccharide-rich liquid byproduct of the process for future use.